Second to diabetes mellitus, thyroid diseases are the most common endocrinopathies seen in pregnancy. Furthermore, thyroid diseases may manifest in the postpartum period, affecting between 5 and 10% of women. Prepregnancy counseling is an important component in the care of women with thyroid disease. It is of paramount importance to achieve euthyroidism before conception; the potential complications of thyroid dysfunction and drug therapy during pregnancy should be discussed at length with the future parents. Thyroid economy is influenced in pregnancy by different factors, among them human chorionic gonadotropin (hCG), changes in thyroxne-binding globulin, and by iodine demands. High levels of hCG or alterations in its structure may stimulate the maternal thyroid gland, affecting not only the interpretation of thyroid tests (as seen in twin pregnancies) but producing clinical or subclinical hyperthyroidism (Hydatidiform mole and hyperemesis gravidarum). Hypothyroidism may be diagnosed in pregnancy for the first time; women on chronic thyroid therapy may need an increase in the dose of thyroid medication because of the increased demands during pregnancy. Rapid correction of hypothyroidism is imperative to prevent maternal and fetal neonatal complications such as pregnancy-induced hypertension (PIH), prematurity, and in some cases fetal distress. The natural history of thyroid cancer is not affected by pregnancy. The work-up and treatment of thyroid nodules may be postponed until after delivery without jeopardizing maternal health. If surgery is contemplated, it is relatively safe to perform it before 24 weeks of gestation. The incidence of spontaneous abortions is increased in women with chronic thyroiditis, as is the development of postpartum thyroiditis. A team approach in the care of women with thyroid disease should include the combined efforts of the obstetrician, endocrinologist, and neonatologist. (C) 1999 Lippincott Williams & Wilkins, Inc.